Currently, it is known practice for a user to communicate data using his or her cell phone over a radiofrequency communication link via a terrestrial telecommunications network, for example a 2G, 3G or 4G network.
When the user is in a motor vehicle, the quality of the radiofrequency communication link may be significantly negatively affected, in particular if the vehicle is moving.
To partly overcome this drawback, it is known practice to fit a motor vehicle with an exterior relay antenna, mounted for example on the roof of the vehicle, allowing the transmission of the radiofrequency communication link between the base stations of the terrestrial telecommunications network and the cell phone of the user located in the passenger compartment of the vehicle to be improved. However, it is been observed that such an antenna may turn out not to be sufficient to effectively relay the communication over the radiofrequency communication link.
Thus, from one known solution for a motor vehicle 1A illustrated in FIG. 1, in order to allow communication between a user device 2 located in the vehicle 1A and a base station 3 of the terrestrial telecommunications network via an exterior relay antenna 4 mounted on the exterior of the vehicle 1A, it is now known practice to mount an interior relay antenna 5 that is connected by wire to the exterior relay antenna 4 inside the passenger compartment of the vehicle 1A. The system thus has three communication links: a radiofrequency communication link L1 between the base station 3 and the exterior relay antenna 4; a wired communication link L2 between the exterior relay antenna 4 and the interior relay antenna 5; and a radiofrequency communication link L3 between the interior relay antenna 5 and the user device 2.
Since the power of the signals received from the base station 3 decreases with increasing distance between the base station 3 and the exterior relay antenna 4, it is preferable for them to be amplified before they are transmitted to the interior relay antenna 5 and then on to the user device 2. To achieve this, it is known practice to use a signal amplifier module, referred to as a compensator 6, which is electrically connected between the exterior relay antenna 4 and the interior relay antenna 5. As is known, this compensator 6 also allows a diagnostic module 7 linked to the interior relay antenna 5 to diagnose the status of the communication link L2 linking the exterior relay antenna 4 and the interior relay antenna 5 in order to detect an operating fault on said communication link L2.
To achieve this, still with reference to FIG. 1, the compensator 6 includes a variable resistor RV, the value of which reflects the status of this communication link L2. By way of example, a first range of values of this variable resistor RV may indicate that the communication link L2 circuit is open between the interior relay antenna 5 and the compensator 6, a second range of values may indicate that the communication link L2 circuit is open between the compensator 6 and the exterior relay antenna 4, a third range of values may indicate that the communication link L2 circuit has shorted between the interior relay antenna 5 and the compensator 6, a fourth range of values may indicate that the communication link L2 circuit has shorted between the compensator 6 and the exterior relay antenna 4, and so on.
The value of the variable resistor RV is defined with respect to a first ground M1, for example the ground of the battery of the vehicle 1A, but it is measured by the diagnostic module 7 located at the interior relay antenna 5. For this, the diagnostic module 7 comprises an electrical circuit including an analog-to-digital converter 7-1, a resistor R1, a capacitor C and a microcontroller 7-2.
The resistor R1 is connected both to a voltage source Vcc and to the input E of the analog-to-digital converter 7-1. The capacitor C is connected both to the input E of the analog-to-digital converter 7-1 and to a second ground M2, this second ground M2 being different from the first ground M1, in particular for safety reasons. The analog-to-digital converter 7-1 converts the analog voltage that it receives as input E (i.e. the voltage V1 defined across the terminals of the capacitor C between the input E of the analog-to-digital converter 7-1 and the second ground M2) into a digital value that can be used by the microcontroller 7-2. The microcontroller 7-2 determines the status of the communication link L2 according to the digital value delivered by the analog-to-digital converter 7-1.
In this configuration, the value of the variable resistor RV is measured by the diagnostic module 7 with respect to the second ground M2. However, since the diagnostic module 7 is not aware of the difference in potential between the first ground M1 and the second ground M2, an error in the measurement of the value of the variable resistor RV, and hence in the digital value delivered by the analog-to-digital converter 7-1 to the microcontroller 7-2, results.
Such a measurement error may give rise to a diagnostic error in the case that the measured value of the variable resistor RV is close to two ranges of values corresponding to two different statuses of the communication link L2 and the diagnostic module 7 designates the range of values that does not actually correspond to the status of the communication link L2.
One obvious solution would consist in determining the potential value of the first ground M1 and in communicating this value to the diagnostic module 7, but such a solution has proven to be both complex and costly.